Archive for January, 2015

Why Your Winter Fuel Economy Stinks

Sunday, January 25th, 2015

Airdrie Mechanic Fuel Economy Tips

Recently, we’ve had a lot of customers come in concerned about poor fuel economy. Sometimes there are repairs or adjustments that we can make to improve their mileage, and sometimes it’s the customer’s expectations that need a little tweak.

We could write an entire article on why your car will never match it’s EPA advertised fuel economy ratings – the “highway” test is done at an average speed of 77 km/h, for example – but there’s another reason why your mileage might be worse right now: it’s winter! The US EPA says you can expect between 10-22% lower fuel economy, depending on how you drive, at -10 degrees vs +25 celsius. Let’s explore the reasons why your car gets so thirsty this time of year.

  • The fuel itself. Gasoline in its liquid form doesn’t really burn. The engine needs to vaporize the fuel, usually by injecting a very fine mist at very high pressures. Unfortunately, gasoline and diesel don’t vaporize as easily at cold temperatures, so fuel manufacturers produce “winter blends” that vaporize more easily to prevent hard starting and rough running. However, this fuel has less heat energy available and produces less power to propell your vehicle, resulting in lower mileage.
  • It’s a mess out there. Driving for winter road conditions naturally burns more fuel. Pushing through snow and ice demands extra energy, as does using four wheel drive more often. You may even be forced to slow down sooner, instead of being able to coast to faraway stops.
  • Your tires. The cold weather, and repeated warm days/cold nights cause your tires to lose pressure more quickly. As we know, low tire pressure increases rolling resistance and lowers fuel economy. Even when your tire pressure is OK, the rubber tires themselves are still stiffer in the cold. This means they don’t flex as easily and rolling resistance is still increased.
  • More electrical load on the engine. Any electrical current that you use has to be produced by the alternator, which puts more load on the engine. In the winter, we’re using our rear window defroster (which is one of the largest loads in the vehicle), the heater, heated seats/steering wheels, etc. We’re probably using the windshield wipers more, too. And if you think these loads might be balanced out by air conditioning use in the summer, consider this: Whenever your heater controls are in the “window defrost” or “floor/window mix” modes, your air conditioning compressor is actually running in order to dry the air and avoid fogging up your windows.
  • Colder air. Colder air is denser, and this affects your vehicle in a number of ways. Because of the aforementioned fuel vaporization issues in the cold, your car actually takes steps to avoid running rough, including running a “richer” air fuel ratio. This means injecting a little extra fuel because it knows that not every bit of fuel that goes through the engine will actually get burned. Denser air also provides more aerodynamic resistance, holding your car back and requiring more fuel to push through. Density is the reason it’s harder to walk through water in a swimming pool than through the air.
  • Your fluids are thicker. Average lubricant viscosity is higher when it’s cold, which means your engine oil, transmission fluid and differential gear oils are all thicker. This translates into extra drag on the gears, bearings, chains etc throughout your powertrain, which means wasted energy. Even your power steering fluid viscosity is higher, placing extra drag on the power steering pump (and the engine).
  • Idle time. This one is huge. 15 minutes of voluntary idle time every day can burn 1.5 cups of fuel for cars, and 3 cups for trucks. This translates to 3-5 litres over the course of a week, or a loss of between 20-40 km per tank for the average commuter who fills up once per week. Double the idle time, (let’s say you warm up your car in the morning, then again before you leave work in the afternoon) and this loss becomes 40-80 km per tank.


So, with all these factors working against you, what can you do to improve your winter fuel economy as much as possible? Here are some ideas:

  • Idle your engine less. 30 seconds of idling is all your engine needs before it’s “safe” to drive, even at very cold temperatures. Just go easy on it until your temperature gauge starts climbing.
  • Use your block heater. Besides reducing engine wear, your block heater allows your engine to reach operating temperature more quickly where it isn’t running a “rich” air/fuel ratio. This will also allow you to reduce warm-up idle time and heater usage. It even warms up thick engine oil and gets it pumping more freely.
  • Replace that worn thermostat. Does your coolant temperature gauge reach its normal level in the cold, or does it struggle to climb as the mercury falls? If your engine isn’t warming up as normal, it’s more than just an annoyance; it’s costing you money in the form of extra fuel that must be burned. You’ll enjoy the improved heater output from replacing that old thermostat, too!
  • Use your manufacturer’s recommended winter grade of oil. Did you know that most car manufacturers recommend different “weights” of oil, depending on what time of year it is?
  • Use synthetic oil. Many vehicles already spec full synthetic oil, and you know that we adhere to those recommendations when they apply. But if your car doesn’t call for synthetic oil, you can still use it and enjoy the reliability and fuel economy benefits – which are the most aparent in cold weather.
  • Say no to “universal” fluids. The most common area where multi-vehicle transmission fluids, power steering fluids, etc don’t meet your vehicle’s specified requirements is their “low temperature pour point” spec. This means they’re thicker than they should be when cold, causing extra drag and potentially damaging your vehicle.
  • Check your tire pressures regularly.